Ancient mixed genomes built the first division-ready eukaryotes

early eukaryotes – Research on the earliest complex cells points to a surprising sequence: internal eukaryotic machinery was already in place—yet key genes for cell division were largely missing. The genome record suggests eukaryotes assembled from a complex mix of archaeal and

Inside the first complex cells, the groundwork looked complete. Protein trackways already threaded through the interior, guided by motor proteins that move cargo within the cell. Lysosomes and peroxisomes were there to digest proteins and handle cellular chemistry. The essentials of eukaryotic life also appeared to be running—DNA replication and RNA production, along with basic metabolism.

But one major piece of the life cycle wasn’t yet present: the sets of genes used to determine when a cell should divide and manage the events needed for that to happen. The researchers suggest this gap may point to how cell division could have begun—not as a fully orchestrated process from day one. but as something initially constrained by metabolic concerns.

The question then becomes how those early genomes ended up so mixed. Roughly a third of gene groups appear to be distinct to eukaryotes and don’t have equivalents in other kingdoms. Some of those may have been present in the lineage that produced the last common ancestor of eukaryotes. Others may have been generated before eukaryotes truly started to diversify and branch out.

Beyond that eukaryote-specific fraction. many genes fit a broad origin story: a large share of the other genes came from either Asgard archaea or Alphaproteobacteria. But the analysis didn’t stop there, and it didn’t read like a simple two-source inheritance. The researchers found roughly equal contributions from two other bacterial groups—Planctomycetota and Myxococcota.

What makes that notable is not just the identity of those groups, but their context. All of the bacterial groups involved are diverse and relatively common, especially compared with the Asgard archaea. The researchers said these results held up across each of three different choices of genes in their analysis. making it less likely the pattern is an artifact.

They also saw small contributions from a range of different bacterial groups. Yet within that broader noise, species from the group of viruses that includes giant viruses contributed more than any single bacterial group.

The team went further and estimated the timing of when different gene groups were introduced. Asgard archaea represented the earliest contribution, consistent with expectations. But there was a bacterial lineage that introduced a lot of genes before mitochondria were present. A second group then made a major contribution afterward.

The researchers say the sequence fits a scenario in which eukaryotes evolved within a microbial mat—an environment where many species stay in close proximity for long periods and may rely on each other for certain metabolites. In that kind of crowded neighborhood, genetic exchange doesn’t have to wait for one dramatic leap. Instead, it can happen as conditions change, with different lineages leaving different marks at different stages.

Taken together. the story the data tell is less about a clean. single origin and more about assembly over time—internal cellular complexity appearing without the full genetic toolkit for division. and gene inputs arriving from multiple microbial sources in a staggered pattern that runs alongside major internal transformations like the appearance of mitochondria.

eukaryotes cell division genomes Asgard archaea Alphaproteobacteria Planctomycetota Myxococcota giant viruses mitochondria microbial mat lysosomes peroxisomes motor proteins